Reduction of metal oxides in an electrolytic cell

ABSTRACT

A method of reducing a titanium oxide in a solid state in an electrolytic cell which includes an anode, a cathode formed at least in part from the titanium oxide, and a molten electrolyte which includes cations of a metal that is capable of chemically reducing the cathode titanium oxide, which method includes operating the cell at a potential that is above a potential at which cations of the metal that is capable of chemically reducing the cathode titanium oxide deposit as the metal on the cathod, whereby the metal chemically reduces the cathode titanium oxide, and which method is characterised by refreshing the electrolyte and/or changing the cell potential in later stages of the operation of the cell as required having regard to the reactions occurring in the cell and the concentration of oxygen in the titanium oxide in the cell in order to produce high purity titanium.

[0001] The present invention relates to reduction of metal oxides in anelectrolytic cell.

[0002] The present invention was made during the course of an on-goingresearch project on the electrolytic reduction of titania (TiO₂) carriedout by the applicant.

[0003] During the course of the research project the applicant carriedout experimental work on an electrolytic cell that included a graphitecrucible that formed an anode of the cell, a pool of molten CaCl₂-basedelectrolyte in the crucible, and a cathode that included solid titania.

[0004] One objective of the experimental work was to reproduce theresults reported in International application PCT/GB99/01781(Publication no. WO99/64638) in the name of Cambridge UniversityTechnical Services Limited and in technical papers published by theinventors.

[0005] The Cambridge International application discloses two potentialapplications of a discovery in the field of metallurgicalelectrochemistry.

[0006] One application is the direct production of a metal from a metaloxide.

[0007] In the context of this application, the “discovery” is therealisation that an electrolytic cell can be used to ionize oxygencontained in a metal oxide so that the oxygen dissolves in anelectrolyte. The Cambridge International application discloses that whena suitable potential is applied to an electrolytic cell with a metaloxide as a cathode, a reaction occurs whereby oxygen is ionised and issubsequently able to dissolve in the electrolyte of the cell.

[0008] European patent application 9995507.1 derived from the CambridgeInternational application has been allowed by the European PatentOffice.

[0009] The allowed claims of the European patent application inter aliadefine a method of electrolytically reducing a metal oxide (such astitania) that includes operating an electrolytic cell at a potentialthat is lower than the deposition potential of cations in theelectrolyte.

[0010] The Cambridge European patent application does not define what ismeant by deposition potential and does not include any specific examplesthat provide values of the deposition potential for particular cations.

[0011] However, submissions dated 2 Oct. 2001 to the European PatentOffice by the Cambridge patent attorneys, which pre-dated the lodgementof the claims that were ultimately allowed, indicate that they believethat the decomposition potential of an electrolyte is the depositionpotential of a cation in the electrolyte.

[0012] Specifically, page 5 of the submissions state that:

[0013] “The second advantage described above is achieved in part throughcarrying out the claimed invention below the decomposition potential ofthe electrolyte. If higher potentials are used then, as noted in D1 andD2, the cation in the electrolyte deposits on the metal or semi-metalcompound. In the example of D1, this leads to calcium deposition andtherefore consumption of thin reactive metal . . . During operation ofthe method, the electrolytic cation is not deposited on the cathode”.

[0014] Contrary to the findings of Cambridge, the experimental workcarried out by the applicant has established that it is essential thatthe electrolytic cell be operated at a potential that is above thepotential at which Ca⁺⁺ cations in the electrolyte can deposit as Cametal on the cathode.

[0015] Specifically, as a consequence of the experimental work, theapplicant has invented a method of reducing a metal oxide such astitanium oxides in a solid state in an electrolytic cell which includesan anode, a cathode formed at least in part from the metal oxide, and amolten electrolyte which includes cations of a metal that is capable ofchemically reducing the cathode metal oxide, which method includes astep of operating the cell at a potential that is above a potential atwhich cations of the metal that is capable of chemically reducing thecathode metal oxide deposit as the metal on the cathode, whereby themetal chemically reduces the cathode metal oxide.

[0016] The above method is described in Australian provisionalapplication PS3049 in the name of the applicant lodged on 20 Jun. 2002,and the disclosure in the patent specification lodged with theapplication is incorporated herein by cross-reference.

[0017] In addition to the above, the experimental work (and associatedtheoretical analysis work) carried out by the applicant has determined anumber of important factors that play a role in the actual reductionprocess.

[0018] The relevant experimental data indicates that (i) Cl₂ gas isremoved at the anode of the electrolytic cell at potentials well belowthe theoretical decomposition potential of the electrolyte CaCl₂, (ii)Ca_(X)Ti_(Y)O_(Z), is present at the cathode during some stages of theelectrolysis, and (iii) CaO is formed in the molten electrolyte bath.

[0019] In view of the above, the applicant has concluded that a numberof steps are involved in the method of reducing titanium oxides and thatsome of these steps are represented by reactions (1) to (8) mentionedbelow. Reactions (1) to (8) relate to reduction of titanium oxides usingan electrolytic cell with CaCl₂ (containing O anions) as the electrolyteand a graphite anode, with their standard potentials at 950° C.

CaCl₂+3TiO₂=CaTiO₃+Cl₂(g)+Ti₂O₃   (1)

[0020] E°_(950C)=−1.45 V

CaCl₂+2TiO₂=CaTiO₃+Cl₂(g)+TiO   (2)

[0021] E°_(950C)=−1.63 V

CaCl₂+0.5TiO₂=CaO+Cl₂(g)+0.5Ti   (3)

[0022] E°_(950C)=−2.4 V

CaTi0₃+C=CaO+TiO+CO(g)   (4)

[0023] E°_(950C)=−0.86 V

CaTi0₃+2C=CaO+Ti+2CO(g)   (5)

[0024] E°_(950C)=−0.96 V

Ti₂O₃+C=2TiO+CO(g)   (6)

[0025] E°_(950C)=−0.58 V

TiO+C=Ti+CO(g)   (7)

[0026] E°_(950C)=−1.07 V

[O]Ti+C=CO(gas)   (8)

[0027] Reactions (1) to (8) are not an exhaustive list, of the possiblereaction and other reactions can take place. Specifically, the applicantsuspects that other reactions, involving titanium suboxides, representedby the formula Ti_(n)O_(2n−1), and calcium titanates, represented by theformula CaTi_(n)O_(3n+1), can take place.

[0028] The potential of reaction (8) in particular varies with theconcentration of oxygen in titanium. The following graph illustrates thevariation of potential with concentration of oxygen in titanium in acell operating at 950° C. The graph was prepared by the applicant usingpublished data.

[0029] It is clear from the graph that reaction (8) requires higherpotentials at lower concentrations of oxygen and thus there is increasedresistance to oxygen removal as the oxygen concentration decreases.

[0030] The solubility of different titanium oxides in CaCl₂ is not takeninto consideration in the calculation of the potentials for reactions(1) to (8). The significance of this is that some of reactions (1) to(8) may take place at potentials that are higher or lower than thepotentials stated above at the stated temperature of 950° C.

[0031] For example, reduced activity of TiO will reduce the value of thepotentials of reactions (2), (4) and (6) (i.e. make the potentials morepositive) and at the samp time will increase the potential of reaction(7) (i.e. make it more negative).

[0032] In view of the above, the applicant has realised that it islikely to be extremely difficult to reduce titanium oxide in anelectrolytic cell to titanium (αTi) of high purity, i.e. lowconcentration of oxygen (no more than 100 ppm oxygen) in a single stageoperation.

[0033] Specifically, the applicant has realised that it is necessary torefresh the electrolyte and/or to change cell potential in a later stageor in later stages of the operation of the electrolytic cell in order toreduce titanium oxide in an electrolytic cell to α titanium of highpurity, ie low concentration of oxygen.

[0034] According to the present invention there is provided a method ofreducing a titanium oxide in a solid state in an electrolytic cell whichincludes an anode, a cathode formed at least in part from the titaniumoxide, and a molten electrolyte which includes cations of a metal thatis capable of chemically reducing the cathode titanium oxide, whichmethod includes operating the cell at a potential that is above apotential at which cations of the metal that is capable of chemicallyreducing the cathode titanium oxide deposit as the metal on the cathode,whereby the metal chemically reduces the cathode titanium oxide, andwhich method is characterised by refreshing the electrolyte and/orchanging the cell potential in later stages of the operation of the cellas required having regard to the reactions occurring in the cell and theconcentration of oxygen in the titanium oxides in the cell in order toproduce high purity titanium (αTi).

[0035] The term “high purity” is understood to mean that theconcentration of oxygen is no more than 100 ppm in the titanium.

[0036] In effect, the present invention is concerned with selecting theoperating conditions of the cell, including cell potential and/orelectrolyte composition, during various stages of the operation in thecell having regard to the reactions that take place in the cell. Theapplicant envisages at this stage that commercial operations will be atconstant currant and that it may not be possible to achieve voltagesrequired to remove oxygen to very low levels because of compositionchanges in the electrolyte. In these circumstances, refreshing and orchanging the electrolyte composition is important in order to produce ahigh purity α titanium.

[0037] The above-described method makes it possible to produce titaniumof high purity with respect to oxygen in an electrolytic cell andwithout refining or otherwise processing the titanium outside theelectrolytic cell.

[0038] The method may include refreshing the electrolyte by adding newelectrolyte to the existing electrolyte or otherwise adjusting thecomposition of the electrolyte.

[0039] In addition, the method may include carrying out the method in aseries of electrolytic cell and successively transferring the partiallyreduced titanium oxide to each of the cells in the series.

[0040] The composition of the electrolyte in each cell may be selectedhaving regard to the reactions occurring in the cell and theconcentration of oxygen in the titanium oxide in the cell.

[0041] The cell potential may be changed at different stages in themethod on a continuous or a step-change basis.

[0042] Preferably the metal deposited on the cathode is soluble in theelectrolyte and can dissolve in the electrolyte and thereby migrate tothe vicinity of the cathode titanium oxide.

[0043] It is preferred that the electrolyte be a CaCl₂-based electrolytethat includes CaO as one of the constituents of the electrolyte.

[0044] In such a situation it is preferred that the cell potential beabove the potential at which Ca metal can deposit on the cathode, i.e.the decomposition potential of CaO.

[0045] The decomposition potential of CaO can vary over a considerablerange depending on factors such as the composition of the anode, theelectrolyte temperature and electrolyte composition.

[0046] In a cell containing CaO saturated CaCl₂ at 1373K (1100° C.) anda graphite anode this would require a minimum cell potential of 1.34V.

[0047] It is also preferred that the cell potential be below thedecomposition potential of CaCl₂.

[0048] In a cell containing CaO saturated CaCl₂ at 1373K (1100° C.) anda graphite anode this would reqtuire that the cell potential be lessthan 3.5V.

[0049] The decomposition potential of CaCl₂ can vary over a considerablerange depending on factors such as the composition of the anode, theelectrolyte temperature and electrolyte composition.

[0050] For example, a salt containing 80% CaCl₂ and 20% KCl at atemperature of 900K (657° C.), decomposes to Ca (metal) and Cl₂ (gas)above 3.4V and a salt containing 100% CaCl₂ at 1373K (1100° C.)decomposes at 3.0V.

[0051] In general terms, in a cell containing CaO—CaCl₂ salt (notsaturated) at a temperature in the range of 600-1100° C. and a graphiteanode it is preferred that the cell potential be between 1.3 and 3.5V.

[0052] The CaCl₂-based electrolyte may be a commercially availablesource of CaCl₂, such as calcium chloride dihydrate, that partiallydecomposes on heating and produces CaO or otherwise includes CaO.

[0053] Alternatively, or in addition, the CaCl₂-based electrolyte mayinclude CaCl₂ and CaO that are added separately or pre-mixed to form theelectrolyte.

[0054] It is preferred that the anode be graphite or an inert anode.

[0055] The cell may be of the type disclosed in the drawings of thepatent specification lodged with Australian provisional applicationPS3049.

1. A method of reducing a titanium oxide in a solid state in anelectrolytic cell which includes an anode, a cathode formed at least inpart from the titanium oxide, and a molten electrolyte which includescations of a metal that is capable of chemically reducing the cathodetitanium oxide, which method includes operating the cell at a potentialthat is above a potential at which cations of the metal that is capableof chemically reducing the cathode titanium oxide deposit as the metalon the cathode, whereby the metal chemically reduces the cathodetitanium oxide, and which method is characterized by refreshing theelectrolyte and/or changing the cell potential in later stages of theoperation of the cell as required having regard to the reactionsoccurring in the cell and the concentration of oxygen in the titaniumoxides in the cell in order to produce high purity titanium (αTi). 2.The method defined in claim 1 wherein the metal deposited on the cathodeis soluble in the electrolyte and can dissolve in the electrolyte andthereby migrate to the vicinity of the cathode titanium oxide.
 3. Themethod defined in claim 1 or claim 2 wherein the electrolyte is aCaCl₂-based electrolyte that includes CaO as one of the constituents ofthe electrolyte.
 4. The method defined in claim 3 wherein the cellpotential be above the potential at which Ca metal can deposit on thecathode, i.e. the decomposition potential of CaO.
 5. The method definedin claim 3 or claim 4 wherein the cell potential is below thedecomposition potential of CaCl₂.
 6. The method defined in any one ofclaims 3 to 5 wherein at a temperature in the range of 600-1100° C. anda graphite anode the cell potential is between 1.3 and 3.5V.
 7. Themethod defined in any one of claims 3 to 6 wherein the CaCl₂-basedelectrolyte is a commercially available source of CaCl₂, such as calciumchloride dihydrate, that partially decomposes on heating and producesCaO or otherwise includes CaO.
 8. The method defined in any one ofclaims 3 to 7 wherein the CaCl₂-based electrolyte includes CaCl₂ and CaOthat are added separately or pre-mixed to form the electrolyte.
 9. Themethod defined in any one of the preceding claims wherein the anode isgraphite or an inert anode.